Simultaneous triple aperture radar

ABSTRACT

An improved radar system and technique are disclosed for use in detecting and tracking moving or stationary targets within the antenna field of view. Precise correction of doppler induced location errors is provided by the use of raw sensor data. Return signals are doppler processed, phase shifted and compared in a manner which preserves the angle of arrival of a moving target irrespective of the boresight direction. Stationary target return signals are constructively combined so as to augment the target signal gain independent of the antenna boresight direction.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of copending application Ser.No. 263,924, filed May 15, 1981, now abandoned, turn is a continuationof application Ser. No. 090,570, filed Nov. 2, 1979, now abandoned.

The present invention relates to radar controlled weapons systems and,more particularly, to a method and apparatus for accurately positioninga moving or stationary target on a radar display as to precisely guideair-to-ground ordnance to a target.

With particular reference to moving target detection, it is well knownthat the reflected radar signal from a moving ground target may exhibita doppler frequency appreciably different from that of ground scatterersin its immediate vicinity due to the additional doppler frequency shiftcaused by the component of target velocity along the antenna boresightdirection. Since a synthetic aperture radar display maps each scattererwith an intensity proportional to its signal strength in a range versusazimuth angle coordinate frame, wherein azimuth angle is scaledaccording to the doppler frequency, a moving target having an additionaldoppler frequency component due to the target radial motion may beincorrectly located in azimuth on the radar display. If the resultingdoppler shift is sufficiently great, e.g., beyond the frequency range ofthe ground clutter being mapped, the target could fall off the displayentirely.

In conventional two-port interferometric radar systems, moving targetinformation is doppler displaced and will typically overlay on a clutterpatch. Depending upon the magnitude of the angular offset of the movingtarget and the strength of the clutter, the resulting azimuth error,using conventional two-port interferometer or sum and difference anglemeasurement techniques for relocating the moving target in azimuth onthe display, can be of the order of one or two milliradians. Since onemilliradian displacement is the equivalent of a one foot error per onethousand feet, this error becomes increasingly significant at the rangestypically intended for airborne radar systems. At a stand-off range often nautical miles, it can be shown that the resultant mislocation wouldbe approximately sixty feet, clearly inadequate in terms of ordnancedelivery.

One approach to providing precise doppler correction in the threeantenna two channel system is described in the commonly assignedcopending U.S. patent application Ser. No. 373,806, filed April 30,1982, now U.S. Pat. No. 4,549,184 entitled "Moving Target OrdnanceControl," which is a continuation-in-part of application Ser. No.272,048, filed June 9, 1981, now abandoned, which in turn is acontinuation of application Ser. No. 010,003, filed Feb. 7, 1979, nowabandoned. The technique proposed therein is based upon sampling thereflected radar signal at each of three antenna ports as each traversesa single line of sight. The time sequenced signals received at a commonlocation are then processed to accurately determine the radial velocityof the target with respect to the boresight direction. Accurate velocitygradient versus interferometer angle gradient information is then usedto reposition the referenced target location as to eliminate the dopplerinduced error.

The present invention is intended to provide an alternative targetdetection system wherein multiple sampling from a common point isunnecessary.

The three port antenna system of the present invention has been found toproduce accuracy approximately an order of magnitude greater than thatof conventional two-port systems. Consequently, at a stand-off distanceof ten nautical miles, the expected mislocation would be approximatelysix feet, a dramatic improvement.

Previous attempts at three-port radar processing systems have met withlimited success due primarily to the signal combining techniques used toderive moving target information from the return energy signals. Forexample, U.S. Pat. No. 3,735,400 discloses a three channelinterferometric synthetic aperture radar system designed for detectingmoving targets wherein the doppler processed signals from the outsideantennas (left and right) are phase shifted so that ground clutter inthese channels is in phase with the center antenna signal. Each of thesesignals is then subtracted from the delayed and doppler processed centerantenna signal to create two different signals which contain movingtargets. A major problem with the disclosed system relates to itsinability to relocate a moving target accurately. The technique employedfor extracting moving target information has resulted in a corruption ofthe system's capability for interferometric angle measurement. Inapparent recognition of this problem, the patentee simultaneously filedand brought to issue U.S. Pat. No. 3,735,399. This second patentrepresents an attempt to overcome the azimuth errors inherent in thefirst system by second order methodology using range rate derivatives.By knowing the movement of the aircraft and the target range fromaperture-to-aperture, the moving target can be more accurately relocatedon the display.

The applicant herein has recognized the problems presented in thepreviously proposed type of system and has disclosed herein a techniquefor accurately relocating moving targets through the use of the rawsensor data, without resort to range rate derivatives which rely onmeasurements of aircraft and target movements from aperture-to-aperture.

It is accordingly a general object of the present invention to providean airborne synthetic aperture radar system which is not accompanied bythe limitations and drawbacks associated with the known systems.

It is a particular object of the present invention to provide a methodand apparatus for detecting and tracking moving and stationary objectswithin the antenna field of view.

It is another object of the present invention to provide a system forrelocating moving targets on a display (or memory) as to ameliorate theeffects of doppler induced location error.

It is a still further object of the present invention to provide atriple synthetic aperture radar system capable of generating highaccuracy moving and stationary target information independent of theboresight direction.

SUMMARY OF THE INVENTION

The foregoing and other objects and advantages which will be apparent inthe following detailed description of the preferred embodiment, or inthe practice of the invention, are achieved by the invention disclosedherein, which generally may be characterized as a triple syntheticaperture radar system. The triple synthetic aperture radar system isoperatively connected to simultaneously receive signals at each of thethree antenna apertures. Signals so received are phase shifted tocompensate for their physical spacing and compared to produce differencesignals representative of a target's characteristic motion and location.

Signals received at the first aperture are phase shifted to effectanticipated clutter coincidence with the second channel, and signalsreceived at the second channel are also phase shifted to effectanticipated clutter coincidence with the third channel. This techniqueallows two difference signals to be generated which accurately preservethe azimuth angle of the moving target irrespective of the boresightdirection.

In the case of stationary targets, summation of signals after phaseshifting the returns serves to reinforce the received signal as toeffect increased stationary target signal gain independent of theantenna boresight direction.

Phase and amplitude imbalances between receiver channels are measured byaveraging the difference signals over the field of view during oneaperture. The data so obtained are used to generate phase and amplitudecorrections which reduce error due to such imbalances in subsequentapertures.

The invention can be adapted for use with conventional two channelreceivers by utilizing multiplexing means for sampling- the receivedsignal at alternating pairs of apertures. By alternating and timesynchronizing the selected apertures, time coincidence may be simulatedto effect triple aperture signal comparison. Moving and stationaryobjects may thereafter be detected and measured as described.

BRIEF DESCRIPTION OF THE DRAWINGS

Serving to illustrate an exemplary embodiment of the invention are thedrawings of which:

FIG. 1 is an illustration of a moving target mislocation problemaddressed by the present invention:

FIG. 2A is a block diagram representative of the simultaneous triplesynthetic aperture radar system of the present invention:

FIG. 2B is a block diagram illustrating in detail the moving targetdetector of FIG. 2A.

FIG. 2C is a block diagram illustrating in detail the moving targetprocessor of FIG. 2A;

FIG. 2D is a block diagram illustrating in detail the fixed targetprocessor of FIG. 2A;

FIG. 3 is a block diagram of a multiplexed two channel version of thepresent invention;

FIG. 4 is a vector diagram illustrating conventional phase shiftingtechniques; and

FIG. 5 is a vector diagram illustrating the phase shifting technique ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In a typical scenario for air-to-ground weaponry, a missile or glidebomb is maneuvered to the start of a near-vertical terminal trajectoryby inertial or radio location mid-course guidance techniques. In theterminal guidance phase, an interferometer antenna system is used inconjunction with a synthetic aperture radar and signal processor toperform measurements of the relative range and azimuth angle between theweapon and a ground target designated by the operator on a highresolution real-time synthetic aperture ground map display.

As a weapon system closes on a target area, accurate target locationinformation is needed in odder to allow precise ordnance delivery. FIG.1 illustrates the effect of target motion upon the observed targetlocation on a radar display. As shown therein, a target at actuallocation ]1, moving with some radial velocity along the antenna line ofsight, introduces an additional doppler frequency component into thereceived radar signal due to its motion, so as to appear at location 13,at a displayed azimuth angle α with respect to the radar platformvelocity V, where its return signal will be added to that of groundclutter at that location.

It has been noted that the received signal from a ground target that ismoving may have a doppler frequency appreciably different from that ofground scatterers in its immediate vicinity due to the additionaldoppler frequency shift caused by the component of target velocity alongthe line of sight to the aircraft. Since the synthetic aperture radardisplay maps each scatterer with an intensity proportional to its signalstrength in a range versus azimuth angle coordinate frame, whereinazimuth angle is scaled according to doppler frequency, a moving targetmay be incorrectly located in azimuth on the display, and, if itsdoppler shift is sufficiently great; e.g., beyond the frequency range ofground clutter being mapped, could move off the display entirely. Inorder for weapon guidance to be performed to such a moving target, it isessential to properly relocate it at position 11 on the display so thatit could be viewed in the context of its surroundings. This initiallyrequires that its range and antenna sensed azimuth angle be determined.

Conventional techniques, however, are still subject to substantialerrors in display location as well as azimuth angle measurement requiredfor weapon guidance. When the moving target's net doppler frequencyfalls within the main beam clutter frequency spectrum, the dopplerfilter containing the moving target signal also contains a signal fromstationary clutter from a totally different azimuth angle within themain beam. It is the presence of this clutter signal in the targetdoppler filter that produces serious errors in moving target azimuthangle measurement. It can further be seen that errors in antenna sensedazimuth angle measurement will produce corresponding errors in displayrelocation.

FIG. 2A illustrates an exemplary embodiment of the present inventionincluding three aircraft mounted antenna ports (12, 15 and 17) whoserespective phase centers are displaced from one another in thehorizontal plane by a distance D. The antenna ports may be threephysically separate antenna assemblies, or they may be component partsof a single array or other integral antenna assembly. The linearlyspaced antenna ports may lie along a line making any arbitrary anglewith respect to the aircraft's velocity vector. Transmission may be froma single array or other integral antenna assembly.

The receivers 19, 21 and 23 can be three separate receivers as shown inFIG. 2A, or a lesser number using time or frequency multiplexingtechniques as illustrated in FIG. 3. Each receiver is a linear amplifierwhose gain is not a function of signal strength.

Compensation for radar platform motion is performed prior to dopplerfiltering by motion compensation unit 25. Techniques used to effect thiscompensation are described in the aforementioned copending applicationSer. No. 373,806, the disclosure of which is incorporated herein byreference.

Synthetic Array Processors 27, 29 and 31 may also be either separateprocessors or integral portions of a single processing device.Commercially available processing systems such as the Vector ArrayProcessor, Model AP 120B, manufactured by Floating Point Systems, or anequivalent, may be readily adapted to perform the continuous processingfunctions of the present invention.

Processors 27, 29 and 31 output processed signals from the right (R),center (C) and left (L) channels respectively into moving targetdetector 35 and fixed target processor 36. Signals R, C and L representreceived signals which have been amplified, filtered and dopplerprocessed into more useful forms by the redundant receiver and syntheticarray processor combinations.

Moving target detector 35 outputs two clutter cancellation residuals Δ1and Δ2 and a moving target detection discrete when a moving target isdetected in the range/doppler cell being processed. When there is nomoving target in that cell, only the detection discrete so indicating isput out by detector 35. The means and method of moving target detectionare illustrated in detail in FIG. 2B.

Moving target processor 34 determines target position and returnamplitude. The means and method of moving target processing areillustrated in detail in FIG. 2C, where the phase angle between the tworesiduals Δ1 and Δ2 is measured and related to the target's position.The target signal magnitude is derived from one of the residuals, inthis case Δ1.

For a particular range and doppler frequency cell such as is illustratedin 13 of FIG. 1, signals R, C and L, derived from simultaneously sampledreceptions at antenna ports 12, 15 and 17 can be represented as:

    R=Me.sup.jφ.sbsp.o +Se.sup.jθ.sbsp.o

    C=Me.sup.j(φ.sbsp.o.sup.+φ) +Se.sup.j(θ.sbsp.o.sup.+θ)

    L=Me.sup.j(φ.sbsp.o.sup.+2φ) +Se.sup.j(θ.sbsp.o.sup.+2θ)

where:

M is the amplitude of the moving target signal in the dopplerfilter/range cell;

φ_(o) is the phase angle of the moving target signal in the rightchannel;

φ is the moving target signal's phase shift from right to center channeland center to left channel due to the moving target signal'sinterferometric angle of arrival;

S is the clutter (or fixed target) signal amplitude;

θ_(o) is the phase angle of the clutter signal in the right channel: and

θ is the phase shift from right to center channels and center to leftchannel due to the clutter signals' interferometric angle of arrival.

By performing Fast Fourier Transforms within the synthetic arrayprocessors on a quantity of N pulses of received radar signals atwavelength λ and at a given Pulse Repetition Interval PRI, we obtain adoppler filter separation, between filters, of df, where

    df=1/(N·PRI)

Using n to indicate a filter number relative to the zero frequency(frequency at antenna boresight/motion compensation point), n can beexpressed as ##EQU1## where Vg is the radar velocity relative to theground and ψ is the angle from the velocity vector to the SAR motioncompensation point.

In addition, the PRI of the transmitted radar pulse can be controlledsuch that: ##EQU2## where D is the antenna spacing and k is someconstant multiplier.

Substituting the two previous equations yields: ##EQU3## θe is definedas the expected interferometric phase shift between channels due to"fixed" targets (ground return) at each doppler frequency beingprocessed. This is nominally the expected interferometric phasedifference between antennas for ground returns, which for a small d ψand using the above defined relationships can be given as ##EQU4##

Referring to FIG. 2B, the R and C signals are first phase shifted byelectrical phase angle θe derived above. These phase rotations areperformed by phase shifters 37 and 39. Then, clutter cancellationbetween adjacent channels is effected by subtracting the phase shiftedsignals R and C from the original signals C and L, respectively. Adders41 and 43 perform these subtractions. The difference signals Δ1 and Δ2thus created may be represented mathematically as:

    Δ1=C-Re.sup.jθe

    Δ2=L-Ce.sup.jθe

The difference signals Δ1 and Δ2 are combined to form a moving targetreturn signal. The difference signals Δ1 and Δ2 are first added in adder45 to reduce the effect of noise. Magnitude detector and comparator 47compares the magnitude of the combined Δ1 and Δ2 to a detectionthreshold. Sufficiently large differences indicate the presence of amoving target in the cell under consideration, in which case a movingtarget detection discrete triggers further moving target processing byprocessor 34 in FIG. 2A and clutter processing by 26 in FIG. 2D.Differences below threshold trigger fixed target processing startingwith phase shifters 28 and 32 in FIG. 2D.

By expanding the previous equations and assuming θe is approximatelyequal to θ, the difference signals can be represented as:

    Δ1=Me.sup.jφ.sbsp.o (e.sup.jφ -e.sup.jθe)

    Δ2=Me.sup.jφ.sbsp.o e.sup.jφ (e.sup.jφ -e.sup.jθe)

which can be expressed as:

    Δ2=Δ1e.sup.jφ.

Referring to FIG. 2C, switches 40 and 42 are closed when a target isdetected, and phase detector 51 operates to compare difference signalsand thereby identify the angle φ between Δ1 and Δ2, which represents thephase shift due to the moving target's angle of arrival, as previouslydescribed.

The moving target can, therefore, be distinguished from the clutterfrequency spectrum and relocated in its proper azimuth location.

By phase detecting the signals of Δ1 and Δ2, one obtains φ and bysubstituting φ in: ##EQU5## and solving for n, one can determine thetrue doppler filter number of the moving target corresponding to itsactual azimuth position. This is performed in target doppler indexcomputer 53. Computer 53 can also derive the angle off boresight δψ ofthe moving target by solving ##EQU6##

Once the electrical phase angle φ is determined, the signal strength ofthe moving target can be calculated by taking the magnitude of ##EQU7##This is performed by magnitude detector 49.

The computing elements in FIGS. 2B and 2C may form an integral portionof the synthetic array processor or may be externally connected. Any ofa number of general purpose digital signal processors are suitable forthese functions.

The present technique allows for concurrent refinement of the displayedtarget location by providing means for correcting phase or amplitudeimbalances between the antenna and receive channel combinations.

Each antenna port should preferably have identical signal responsepatterns and be uniformly spaced for mathematical convenience. As oneskilled in the art will recognize, the preference for matched signalpatterns can be eased by using the residual signals (Δ1 and Δ2) torecognize irregularities in each pattern and compensating for theseirregularities.

Variations in the response of each antenna port/receiver channelcombination can also be determined by comparing the phase rotatedreceived pulses with the pulses received in the adjacent port. If phaseor amplitude differences between the two representative signals aremeasured over a period of time, a performance error can be computed andused to correct further incoming signals.

One reference to a system for compensating for uncalibrated and timevarying parameters is provided in U.S. Pat. No. 3,993,994, "AdaptiveClutter Cancellation for Synthetic Aperture AMTI Radar," assigned to theUnited States of America as represented by the Secretary of the AirForce. An alternate method better adapted to the present system is asfollows.

Since a relatively small number of range/doppler cells in the SARaperture will contain moving targets, for the large majority of cells,differences Δ1 and Δ2, if any, may be considered as due to phase andamplitude imbalances between channels. An estimate of these imbalancescan be obtained by averaging the quantities Δ1/R and Δ2/L over allrange/doppler cells. The imaginary and real parts of the first averagequantity will represent the phase imbalance and amplitude imbalance,respectively, between the right and center receiver channels. Similarly,the imbalances between the left and center receiver channels will begiven by the imaginary and real parts of the second average quantity.These imbalance corrections can be applied to the raw signals beforeprocessing at 27, 29 and 31 during the next SAR aperture.

Phase and amplitude balancing control may be effected by means ofamplitude and phase imbalance computer 33. Commercially available unitssuch as the AP 120B, previously described, or the Model PDP11-34M,manufactured by Norden/DEC are both suitable for performing thesefunctions.

One advantage of the present invention is that precise control of thepulse repetition frequency is not a requirement, as it would be forArrested Synthetic Aperture Radar (SAR), Displaced Phase Center Antenna(DPCA) or other techniques which form consecutive apertures at a singlepoint in space. Simultaneous sampling allows the PRF to vary at anyfrequency since each channel receives the same signal, regardless of thesignals absolute frequency. Any variation of the PRF would, therefore,effect each channel similarly.

Since the spacial relationships between the antenna ports is known, theexpected phase difference between antennas can be determined andanticipated phase coincidence between adjacent channels can be effected.

Prior triple aperture techniques have been deficient in that there hasbeen proposed no system, using the raw radar signal, which is capable ofaccurate moving and stationary target tracking along any boresightdirection.

By phase rotating each doppler processed received signal according tothe present invention, intrinsic angular information is preserved andmeasured for accurate target location.

FIGS. 4 and 5 illustrate conventional and proposed phase rotatingtechniques, respectively, demonstrating the angular informationderivable from each. In FIGS. 4 and 5, three received signals, L', R'and C' representing the moving target's contribution to resultantphasors R, C and L, are shown having equal signal magnitude and 90°phase difference. By rotating the right and left signals to anticipatedclutter coincidence with the center signal, as in FIG. 4, two differencesignals, Δ1 and Δ2 can be generated. The angle between Δ1 and Δ2 isdefined as β_(o).

FIG. 5 by comparison shows a technique for phase shifting the rightsignal only into expected clutter coincidence with the center signal.The center signal is phase shifted into expected clutter coincidencewith the left signal. The angle between the two difference signals islabeled β₁.

In FIG. 5, it can be seen that β₁ is the same angle as that between Land C (β). This relationship holds true regardless of the boresightdirection or the target motion. In FIG. 4, the angle β_(o) is not thesame angle as that between R and C (β). Although for some geometries, βmay be the same angle as that between adjacent channel signals, it wouldbe mere coincidence.

This mechanization difference provides the present invention thecapability to perform interferometric phase angle (and hence angle ofarrival) measurement on the moving target without requiring target rangerate information and other navigational signals. The inventor herein hasrecognized the cause of corruption of angular information inconventional systems and conceived the proposed system overcoming theseproblems.

When a moving target is found, switches 14, 16 and 18 of FIG. 2D areclosed, and fixed target (or clutter) processing consists in solving theequations representing phasors R, C and L to obtain the clutter signalcomponent S. This is done in stationary clutter component processor 26of FIG. 2D by calculating the absolute magnitude of either ##EQU8##

The moving target's signal magnitude M having been determined andrelocated in doppler filter n, i.e., into position 11 of FIG. 1,stationary signal magnitude S is now placed into position 13, i.e., inthe doppler filter being processed.

In the absence of a moving target, switches 20, 22 and 24 are closed andthe simultaneous input signals R and L are phase rotated by phaseshifters 28 and 32 to anticipated vector coincidence with C and thenadded to C by adder 30 so that the signals vectorially reinforce eachother. By effecting phase coincidence before signal summing, the presentinvention allows high gain mapping independent of the target's anglerelative to boresight. The final product will be a signal havingapproximately triple the strength of the received impulse on anyparticular antenna/receiver channel without requiring any increase inoperating power. For this reason, the output of processor 26 in FIG. 2Dis multiplied by three in multiplier 50 to balance display magnitudes ofall doppler filters. The outputs of processor 36 may then be displayedin Display and Target Cursor Processor 38 in FIG. 2A.

The moving target's signal magnitude and doppler index and thestationary clutter signal magnitude and doppler index are communicatedto the display and target cursor processor 38 which generates a radarrange versus doppler display.

FIG. 3 depicts an exemplary time multiplexing scheme for mechanizing thesystem with only two receivers. The multiplexing technique permits theinvention to be compatible with many receiver systems currently inaircraft use. Various switching and phase rotating sequences can be usedas long as the resulting angle between difference signals indicates thereceived signal angle of arrival.

On the first and subsequent odd number pulses of an aperture, the rightand center channels are processed in synthetic array processors 54 and56. On the second and subsequent even numbered pulses of an aperture,the right and left channels are processed in synthetic array processors55 and 57. Phase detector 58 determines the phase shift β between oddand even pulses, as received in the right channel. Detector 58 outputsphase β by which the phase shifter 59 rotates the left channel signal totime (phase) synchronize the left channel signal with the right andcenter channel signals in processors 54 and 56.

In practice, processor 55 and phase detector 58 may not be required andmay be replaced by a phase computer since the pulse-to-pulse phasedifference can be estimated knowing the average PRF during the apertureand the frequency fd of the particular doppler filter being processed.

The electrical phase difference β between odd and even arrays is givenby ##EQU9##

Signals R, C and L obtained in FIG. 3 can then be further processed asshown in FIG. 2A.

Stationary target mapping is carried out in the same manner as with thethree receiver system, further including the pulse-to-pulse timesynchronization as just described.

While the invention has been described in one presently preferredembodiment, it is understood that the words which have been used arewords of description rather than words of limitation and that changeswithin the purview of the appended claims may be made without departingfrom the scope and spirit of the invention in its broader aspects.

What is claimed is:
 1. An airborne simultaneous triple syntheticaperture radar system comprising:first, second and third antenna meanslinearly spaced on an aircraft, each of said antenna means beingelectrically connected to a dedicated receive channel processor, first,second and third frequency filter means in electrical communication withsaid first, second and third antenna means, respectively, means forphase shifting the output of said first filter means to effect cluttercoincidence with the output of said second filter means, means for phaseshifting the output of said second filter means to effect cluttercoincidence with the output of said third filter means, signal reductionmeans for combining clutter coincided filter outputs to derive aresidual signal from each pair of clutter coincided signals, signaladdition means for combining the two residual signals so obtained,signal comparison means for comparing the combined residual signals to athreshold and generating a moving target detection indicator signal,signal comparison means adapted to determine phase relationships betweenresidual signals to define the electrical phase angle between them, anddata processing means for deriving the doppler position and angle offboresight of the moving target.
 2. The apparatus as described in claim1, wherein said antenna means include three physically separate antennaassemblies.
 3. The apparatus as described in claim 1, wherein saidantenna means include three component parts of a single antenna array.4. The apparatus as described in claim 1, further including means foraveraging each of the residual signals over time to define amplitude andphase imbalances in the antenna means and receive channel processors. 5.The apparatus as defined in claim 1, further including signal dividingmeans for extracting defined moving target information from the filteroutput signals, phase shifting means for effecting clutter coincidencebetween the non-moving target portion of each filter output signal, andsignal summing means for combining phase coincided portions of adjacentfilter output signals as to define the representative signal amplitudeof objects not in motion relative to a motion compensation point.
 6. Theapparatus as described in claim 4, further including signal balancingmeans for processing said imbalances as to match phase and amplitudecharacteristics of each antenna means and receive channel processorcombination.
 7. The apparatus as defined in claim 5, further includingmeans for combining the non-moving portions of each filter outputsignals to generate a ground map display.